1. Field of the Invention
The present invention relates to a filter having a coupling loop, to a duplexer, and to a communication device.
2. Description of the Related Art
As FIG. 10 shows, a conventional filter 110 comprises a dielectric resonant device 120, metal panels 111 having an external connector 113 that serves as an input-output connecting means and covers open portions of the dielectric resonant device 120, and a coupling loop 112.
The dielectric resonant device 120 includes a frame 121 and a dielectric resonator 122 that are made of ceramic. The frame 121 is shaped like a parallelepiped with two opposing surfaces being open, and is provided with conductors 123 thereon. The dielectric resonator 122 is shaped like a rectangular parallelepiped, and is disposed inside the frame 121 so that its two opposing surfaces are integrated with the frame 121. The metal panels 111 are made of metal, such as iron or a nickel alloy, in order to achieve good electrical conductivity and to make the coefficient of linear expansion thereof the same as that of a dielectric. These metal panels 111 are connected to the conductors 123 of the dielectric resonant device 120, whereby a cavity 130 is formed as a whole.
The coupling loop 112 is made of copper in view of electrical conductivity and rust prevention, and worked into the shape of an L. One end of the coupling loop 112 fits in a hole that is previously formed through the metal panel 111, and is fixed by soldering or the like. The other end of the coupling loop 112 is connected to the external connector 113. Since this other end of the coupling loop 112 is also worked into a corrugated shape, it can, for example, absorb impact that is applied from the side of the external connector 113. This has solved problems, for example, deformation of the coupling loop 112 due to stress from the outside, and separation of the coupling loop 112 from the metal panel 111.
In the filter 110 mentioned above, current applied from the outside flows in the coupling loop 112 via the external connector 113. The current that flows through the coupling loop 112 generates a magnetic field, and this magnetic field couples with the dielectric resonator 122. In this case, the degree of coupling between the coupling loop 112 and the dielectric resonator 122 is adjusted based on the length, thickness, and width of the coupling loop 112 or the distance between the coupling loop 112 and the dielectric resonator 122. Such adjustment of the degree of coupling allows a filter having the required electrical characteristics.
A coupling loop has its own natural frequency, and the natural frequency of a coupling loop in a conventional filter is about 260 Hz. On the other hand, in normal use of the filter, a device itself, in which the filter is incorporated, vibrates with the vibrations applied from the outside. In this case, frequencies ranging from about 5 Hz to 200 Hz are a problem. There is a likelihood that a coupling loop will resonate with the vibrations from the outside. The coupling loop resonates because the frequency of the external vibrations is almost equivalent to the natural frequency of the coupling loop. Although the natural frequency of the conventional coupling loop does not coincide with the frequency of the external vibrations, if it remains about 260 Hz, the attenuation amount is not sufficient near about 200 Hz, which is an unnecessary signal, thereby affecting the filter characteristics to a degree that is not disregarded. As the coupling loop resonates with the external vibrations, the degree of coupling between the coupling loop and the dielectric resonator varies, and the electrical characteristics, such as return loss, are thereby disturbed. Moreover, reliability of the filter is deteriorated.
In order to solve the above problems, it may be possible to further increase the natural frequency of the coupling loop so that the resonance with the external vibrations can be disregarded. Incidentally, the coupling loop can be regarded as having a beam structure. In general, the natural frequency of a beam is expressed by the following formula:
Natural Frequency   f  =            C              l        2              ⁢                  EI        pA            
where C is a constant, l is the length of the beam, E is the Young""s modulus of the beam, I is the second moment of area of the beam, p is the density of the beam, and A is the sectional area of the beam.
Referring to the above formula, it may be possible to reduce the length of the beam in order to increase the natural frequency of the coupling loop. Since the length of the beam has an influence on the degree of coupling with the dielectric resonator, however, it cannot be easily changed. Accordingly, it is good, in practice, to change the bending rigidity of the beam. The bending rigidity of the beam is given by the product of the Young""s modulus and the second moment of area of the material. Therefore, the bending rigidity of the beam can be improved by increasing the Young""s modulus or the second moment of area of the material. Although iron is available as a material having a high Young""s modulus, the use of iron for the coupling loop causes a new problem, that is, thorough rust prevention is required. When the coupling loop is made of iron, in general, intermodulation (IM) is apt to occur, and therefore, the coupling loop is plated with silver. If the silver plate rusts, however, iron appears on the surface thereof, and IM is likely to occur. Although it may also be possible to increase the thickness of the coupling loop in order to increase the second moment of area, this results in an increase in the material cost.
The coupling loop is formed by bending a metal plate into the shape of an L. Therefore, the strength of the bent portion is low, and this leads to a fear that the positional relationship between the coupling loop and the dielectric resonator may change.
Furthermore, one end of the coupling loop on the side of the external connector has been heretofore corrugated so as to absorb impact from the external connector. It is, however, not so easy to corrugate an end of the coupling loop, and costs become high.
The present invention has been made with a view toward solving the above problems. It is accordingly an object of the present invention to provide a filter, a duplexer, and a communication device that are hardly affected by vibrations applied from the outside and that have high reliability.
According to an aspect of the present invention, there is provided a filter having a cavity, an input-output connecting means mounted in the cavity, and a coupling loop connected to the input-output connecting means so as to couple with a magnetic field inside the cavity, wherein the coupling loop has a natural-frequency increasing means for increasing the natural frequency thereof.
According to another aspect of the present invention, there is provided a filter having a cavity, an input-output connecting means mounted in the cavity, and a coupling loop connected to the input-output connecting means so as to couple with a magnetic field inside the cavity, wherein the coupling loop is formed by bending a metal plate, and is provided with a rib extending in a direction that is not in parallel with a bending line.
According to a further aspect of the present invention, there is provided a filter having a cavity, an input-output connecting means mounted in the cavity, and a coupling loop connected to the input-output connecting means so as to couple with a magnetic field inside the cavity, wherein the coupling loop including a section having high rigidity and a curved section having low rigidity, one end of the high-rigidity section is connected to the cavity, the other end thereof is connected to one end of the low-rigidity curved section, and the other end of the low-rigidity curved section is connected to the input-output connecting means.
Preferably, a dielectric resonator is disposed inside the cavity.
Preferably, a rib is formed in a bent portion of the coupling loop.
This structure makes it possible to increase the natural frequency of the coupling loop, and to thereby prevent the coupling loop from resonating with vibrations from the outside. Moreover, it is possible to mechanically reinforce the bent portion of the coupling loop, and to limit the change of degree of coupling, whereby a reliable filter can be provided.
Accordingly, it is possible to easily manufacture at low cost a coupling loop that is connected to an external connector and that absorbs impact applied through the external connector. In addition, it is possible to provide a reliable filter in which change of degree of coupling is prevented.
According to a further aspect of the present invention, there is provided a duplexer including at least two filters, input-output connecting means connected to the filters, and an antenna connecting means commonly connected to the filters, wherein at least one of the filters is a filter of the above-mentioned type.
According to a still further aspect of the present invention, there is provided a communication device including the above-mentioned duplexer, a transmission circuit connected to at least one of the input-output connecting means in the duplexer, a receiving circuit connected to at least one of the input-output connecting means that differs from the input-output connecting means connected to the transmission circuit, and an antenna connected to the antenna connecting means in the duplexer.
According to the above, it is possible to obtain a duplexer and a communication device having stable properties and high reliability.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.